The computation of a downlink (DL) beamforming weighting vector is based on channel information in the downlink direction. In a time division duplex (TDD) system, the channel information in the downlink direction becomes available as long as the uplink (UL) channel information is known. This is true due to the reciprocal nature of the DL and UL TDD channels. However, in a frequency division duplex (FDD) system, this kind of reciprocal characteristic does not exist between the DL and UL channels. As a result, the information about the DL channel must be sent back to a base transceiver station (BTS) explicitly by a mobile station (MS).
In orthogonal frequency division multiplexing (OFDM) and orthogonal frequency division multiple-access (OFDMA) systems, the carriers of OFDM symbols may experience different levels of impairment. Whenever there is a significant change in the channel quality of a sub-carrier, the MS must send channel information back to the BTS explicitly.
In a TDD OFDMA system, the frequency separation between the DL and UL channels might vary from a fraction of a MHz to a few MHz. This is due to the fact that the BTS scheduler assigns a sub-carrier (frequency) to the DL and UL channels dynamically.
For example, in the TDD version of IEEE 802.16 d/e (WiMax) standard, the DL and UL channels both operate in one of the following frequency bands, i.e., 2.5 MHz, 5 MHz, 10 MHz and 20 MHz. The DL channel is divided into sub-carries, any number of which could form a sub-channel. A permutation is designed to minimize the probability of reusing the sub-carriers in adjacent cells.
Depending on which permutation is used, the DL and UL channels may have few or no sub-carriers in common. FIG. 1 is a diagram illustrating an arbitrary assignment of sub-carriers in the UL and DL channels in a two-dimension diagram of time and frequency domains.
In FIG. 1, a radio channel is divided into 24 sub-carriers 110, each of which is represented by an empty block. Nine of the 24 sub-carriers are assigned to a BTS in a cell for downlink traffic. The nine sub-carries are grouped into six sub-channels 120, each of which is represented by a block with dots. Six of the 24 sub-carriers are assigned to an MS for uplink traffic. The six sub-carries are grouped into five sub-channels 130, each of which is represented by a block with horizontal lines. Each of the sub-channels is composed of one or more sub-carriers.
Although the frequency separation between the UL and the DL channels is small, the BTS cannot use UL channel information to estimate the DL channel condition with the traditional methods.
As such, what is desired is a method for computing a DL beamforming weighting vector based on UL channel information in a TDD OFDMA system where there is little or no overlap between the sub-carriers in the UL and the DL channels.